Conventionally, some antenna device capable of corresponding to a plurality of information communication systems using one frequency is used together with a communication module shown in FIG. 19. In FIG. 19, communication module 100 corresponds to both short-range communication system 103 and Wireless-Local Area Network (W-LAN) system 104. In designing such communication module 100, the following points need to be considered:
two systems 103 and 104 use the same frequency band such as 2.4 GHz band; and
these systems are simultaneously used.
In other words, both systems can be simultaneously in a transmitting state or in a receiving state, or one system can be in the transmitting state and the other system can be in the receiving state. In the latter case, a signal from one system works as an interference signal with the other system to significantly increase the bit error rate (BER) of a received signal of the latter system.
For preventing this radio interference, conventionally, a high-frequency filter is directly connected to an antenna to remove signals from the other system. However, two systems 103 and 104 use the same frequency band in communication module 100 in FIG. 19, so that the method of the direct connection cannot be used for rejecting the signals from the other system. In communication module 100, therefore, systems 103 and 104 have respective independent antennas 101 and 102, thereby preventing the radio interference between the systems. An arranging method of two antennas 101 and 102 is thus designed, thereby securing the isolation between the systems.
According to a theoretical calculation in case that two dipole antennas for 2.4 GHz are employed, for example, the interval between both antennas is required to be 320 mm for securing the isolation of 26 dB between the antennas.
Two antennas 101 and 102 are required to be physically separated from each other in the structure discussed above, so that a housing for mounting communication module 100 inevitably increases in size. Further, two positions for mounting the antennas need to be secured in case that two separated antennas 101 and 102 are employed, so that device design is restricted and cost required for the antenna device doubles.